Dichromate oxidation of alcohols

Dichromate oxidation of secondary alcohols produces ketones in good yield, with little additional oxidation. For example, CH,CH2CH(OH)CH3 can be oxidized to CH CH2COCH3. The difference between the ease of oxidation of aldehydes and that of ketones is used to distinguish them. Aldehydes can reduce silver ions to form a silver mirror—a coating of silver on test-tube walls—with Tollens reagent, a solution of Ag1" ions in aqueous ammonia (Fig. 19.3) ... 

This complex, formerly called pyridine perchromate and now finding application as a powerful and selective oxidant, is violently explosive when dry [1], Use while moist on the day of preparation and destroy any surplus with dilute alkali [2], Preparation and use of the reagent have been detailed further [3], The analogous complexes with aniline, piperidine and quinoline may be similarly hazardous [4], The damage caused by a 1 g sample of the pyridine complex exploding during desiccation on a warm day was extensive. Desiccation of the aniline complex had to be at ice temperature to avoid violent explosion [4]. Pyridinium chlorochromate is commercially available as a safer alternative oxidant of alcohols to aldehydes [5], See Chromium trioxide Pyridine Dipyridinium dichromate See Other AMMINECHROMIUM PEROXOCOMPLEXES... 

Adogen has been shown to be an excellent phase-transfer catalyst for the per-carbonate oxidation of alcohols to the corresponding carbonyl compounds [1]. Generally, unsaturated alcohols are oxidized more readily than the saturated alcohols. The reaction is more effective when a catalytic amount of potassium dichromate is also added to the reaction mixture [ 1 ] comparable results have been obtained by the addition of catalytic amounts of pyridinium dichromate [2], The course of the corresponding oxidation of a-substituted benzylic alcohols is controlled by the nature of the a-substituent and the organic solvent. In addition to the expected ketones, cleavage of the a-substituent can occur with the formation of benzaldehyde, benzoic acid and benzoate esters. The cleavage products predominate when acetonitrile is used as the solvent [3]. 

The reaction sequence Is based on the readiness with which a-pinene undergoes oxidation predominantly to the tertiary acetate with Pb(OAc)4.3 4 Dichromate oxidation of the derived alcohol proceeds by way of a second allylic rearrangement to give verbenone without affecting the neighboring stereogenic centers. 

The oxidation of alcohols to the corresponding aldehydes, ketones or acids certainly represents one of the more important functional group transformations in organic synthesis and there are numerous methods reported in the literature (1-3). However, relatively few methods describe the selective oxidation of primary or secondary alcohols to the corresponding aldehydes and ketones and most of them traditionally use a stoichiometric terminal oxidant such as chromium oxide (4), dichromate (5), manganese oxide (6), and osmium or ruthenium oxides as primary oxidants (7). 

Laboratory oxidation of alcohols most often is carried out with chromic acid (H2Cr04), which usually is prepared as required from chromic oxide (Cr03) or from sodium dichromate (Na2Cr207) in combination with sulfuric acid. Ethanoic (acetic) acid is a useful solvent for such reactions ... 

General Procedure for Oxidation of Alcohols to Aldehydes and Ketones with Pyridinium Dichromate (PDC)... 

The oxidation of alcohols with metal dichromates, other than sodium or potassium dichromate, has been little explored. Hydrated zinc dichromate (ZnCr207 3H20)367 368a and ferric dichromate [Fe2(Cr207)3],368b—which are very easy to prepare as stable solids—are able to oxidize alcohols in organic solvents.368 Zinc dichromate is particularly efficient in the transformation of a-hydroxyphosphonates into a-ketophosphonates.369... 

A compound prepared and first described as nicotinium dichromate (NDC) by Palomo et al.,379 was later shown by X-ray-crystal analysis380 to be a betainic mixed anhydride of nicotinic and chromic acid (NACAA). Because of its unique structure, it deserves a close scrutiny of its oxidative properties.381 Replacement of the chloride anion in the quaternary ammonium resin, Dowex 1-X8, for the dichromate anion, leads to a polymer supported dichromate, which is able to make selective benzylic oxidations.382 Finally, poly[vinyl(pyridinium dichromate)] (PVPDC), a polymeric analogue of PDC, must be mentioned whose use in the oxidation of alcohols allows for a very easy work-up.383... 

The selective oxidation of alcohols is widely recognized as one of the most fundamental transformations in both laboratory and industrial synthetic chemistry because the resulting carbonyl compounds serve as important and versatile intermediates for the synthesis of fine chemicals [4]. Many oxidizing reagents, including permanganate and dichromate, have traditionally been employed to achieve this transformation [5]. These stoichiometric oxidants, however, have serious... 

A second-order dependence on both the reductant and acidity was observed in the oxidation of alcohols with butyltriphenylphosphonium dichromate study of MeCD2OH and Me2CDOH indicated the presence of a substantial kinetic isotope effect. The reaction was studied in 19 organic solvents and the rates were correlated with mul-tiparametric equations. The reaction is susceptible to both electronic and steric effects of the substituents. A mechanism involving the formation of a dichromate ester and an a-C-H cleavage has been proposed.4... 

Oxidation of alcohols is normally carried out with Cr(VI) reagents (Chapter 24) but these, like the Jones reagent (Na2Cr2C>7 in sulfuric acid), are usually acidic. Some pyridine complexes of Cr(Vl) compounds solve this problem by having the pyridinium ion (p Ta 5) as the only acid. The two most famous are PDC (Pyridinium DiChromate) and PCC (Pyridinium Chloro-Chromate). Pyridine forms a complex with CrO but this is liable to burst into flames. Treatment with HC1 gives PCC, which is much less dangerous. PCC is particularly useful in the oxidation of primary alcohols to aldehydes as overoxidation is avoided in the only slightly acidic conditions (Chapter 24). 

Oxidation of alcohols. In the presence of catalytic amounts of pyridinium dichromate, this peroxide can oxidize primary and secondary alcohols to the corresponding carbonyl compounds in 70-100% yield. Reactions catalyzed by dichloro-tris(triphenylphosphinc)ruthenium are useful for highly selective oxidation of primary allylic and benzylic alcohols m the presence of secondary ones. 

An alternative tt> the chromium trioxide-pyridine comidex is provided by pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC). These reag ts, now ubiquitous for chromate-based oxidation of alcohols, overcome the hygroscofric nature of the chromium trioxide-i ridine complex and are prepared by a less hazardous procedure both are commercially available as are several other derivative reagents. 

Carboxypyridinium dichromate (isonicotinium dichromate) has also been prepared by Palomo and coworkers. It appears to have very similar properties to nicotinium dichromate, and offers no further advantage for the oxidation of alcohols. 

Corey, E. J., Schmidt, G. Useful procedures for the oxidation of alcohols involving pyridinium dichromate in aprotic media. Tetrahedron Lett 1979,399402. 

Chromic acid, in the form of its anhydride generally, dissolved in glacial acetic add, or as a water solution of potassium dichromate or sodium didiromate acidified with dilute sulphuric add, can also be used as an oxidising agent, not only in the case in hand, but also for the oxidation of alcohols, ketones, etc. In oxidation reactions, two molecules of chromic anhydride (Cr03) give three atoms of oxygen ... 

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